This invention relates to a method for improving the extraction of plutonium with tributyl phosphate. More particularly, it relates to a method for improving the degree to which plutonium ions can be stripped from an organic to an aqueous solution by the removal of radiolytic degradation products of tributyl phosphate which would otherwise form nonstrippable plutonium complexes and precipitates.
Solvent extraction processes are in wide use for the reprocessing of spent fuel from nuclear reactors as well as actinide scrap material generated in the fabrication of nuclear fuel elements. Tributyl phosphate (TBP) in an organic solvent base or diluent, e.g., kerosene or carbon tetrachloride, is often employed to extract the actinides from the fission products or other impurities. This process is generally referred to as the Purex Process and is described in "Reactor Handbook," 2d Edition, Volume II, "Fuels Reprocessing," Stoller and Richards, Interscience, 1961. A more particular process to which the present invention is applicable is described in Chemical Engineering Progress Symposium Series, Vol. 63, No. 80, L. E. Bruns, "Geometrically Favorable Plutonium Scrap Recovery Plant," pp. 156-162, 1967. In order to partition the actinide values, this process relies on the propensity of uranyl nitrate to be held in the organic phase by tributyl phosphate while plutonium-III is stripped into an aqueous phase.
The use of tributyl phosphate as an extractant for uranium and plutonium has a widespread acceptance in the nuclear industry for the past 25 years. This material, nevertheless, suffers from both chemical and radiological degradation when exposed to the radioactive solutions containing extractable actinides. The presence of the degradation products such as the dibutyl phosphate (DBP), monobutyl phosphate (MBP) and phosphoric acid has a detrimental effect upon the extraction process. Among the problems created by the presence of these materials are poor phase separation in the contactor or column with subsequent losses by entrainment, reduced throughput, extraction of fission products along with the actinides and the inability to partition actinides caused by the formation of nonstrippable complexes and precipitates in the organic phase.
Previous techniques to remove degradation products from tributyl phosphate have included carbonate scrub solutions, the use of macrorecticular ion exchange resins and others such as a reduction scrub with hydrofluoric acid. All of these techniques involve the use of extra process equipment and chemicals along with the disposal of the additional radioactive waste. With the use of carbonate scrubs, the concentration of degradation products rises to a level such that the entire organic stream must periodically be discarded. Hydrofluoric acid brings with it the risk of PuF.sub.4 precipitation. All of these treatment techniques are expensive. An ideal solution to these problems would be one which would minimize the formation of these degradation products or permit their removal at an early point in the process.